Dredging apparatus

ABSTRACT

A dredging apparatus comprising a body mounting thruster to direct, in use, a wash of water downwards towards an area of sea bed or the like, connection device to connect the dredging apparatus to a support vessel above the sea bed, said connection device including attitude adjusted to selectively adjust the attitude of the dredging apparatus in a side to side (roll) orientation.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to dredgers for removing sand, silt and likematerial from the river or sea bed and has application, for example toclearing wrecks, and providing trenches in which, for example, pipelinesmay be laid.

2. Description of the Related Art

A suction dredger is the most widely used apparatus for removing suchmaterial, suction being created by a motor and pump unit, somewhat likea vacuum cleaner. However, if used for clearing wrecks, such apparatushas the disadvantage that small and/or lightweight articles from thewreck can also be lifted and, even if a screen is provided in thesuction path, the articles may be small enough to pass through thescreen, or be difficult to extract from the other debris lifted.

A modified form of the suction method, which is used in tidal waters, isto provide a vertical length of pipe above the area to be cleared nearthe lower end of which air is fed under pressure to pass upwardlythrough said pipe. This creates a vacuum, which will act to lift thesand, silt and like material and set it in suspension with the water,whereafter it may be carried away from the area by the tide. This methodis reliable in reducing the possibility of small/lightweight articlesbeing lost, but is time consuming due to the relatively small diameterof the pipe, normally around 0.5 metres, and hence restricted areacovered.

Another method, which can be used in relatively shallow tidal waters,e.g. up to about 10 metres in depth, comprises mooring a tug, ship orother vessel in a fixed position above the area to be cleared anddeflecting the propeller wash downwardly using a suitable guide plate.The wash disturbs the material around the wreck, which material isthereby lifted, set in suspension and carried away from the area by thetide. Apart from the shallow depth, another restriction of this methodis that, for a large wreck, the position of the vessel must be changedprogressively to cover the complete area of the wreck, which isdifficult and time consuming.

EP-A-328 198 disclose a method of dredging in flowing water comprisinglowering a casing of a wing shape downwardly towards the area to becleared, the casing carrying thrust means arranged so that the thrustmeans is directed downwardly, the orientation of the wing casing beingadjusted in the water so that it presents a surface relative to the flowwhich causes a resultant downward vertical component of force tocounteract the upward force provided by the thrust means, the thrustmeans directing a wash of water towards the areas to be cleared so thatthe turbulence created clears the sand, silt or like material coveringthe area.

This method of dredging is particularly useful for providing a trenchacross the sea bed. The wing shape casing is slowly towed along a lineabove the sea bed and the thrust means, which is directed verticallydownwards, excavates a trench in the sea bed of a width which dependsupon the material of the sea bed, its altitude above the sea bed, thepower in the thrusters, its speed over the sea bed, and its pitch angle.In a typical example, the width of trench formed will be of the sameorder as the width of the wing shape casing.

Such a dredger, which is commonly known as a "wing dredger" has beensuccessful in producing a trench of a width sufficient to take apipeline or, alternatively, to flatten an area of sea bed in preparationfor works on the sea bed. However, its usefulness could be increased ifit could, on occasions, provide a trench or clear an area of the sea bedof a greater width. This would allow a single wing dredger to be used ina wider variety of circumstances or alternatively a smaller wing dredgerto be used to provide a trench of a particular width.

One's initial view might be that increasing the thrust with accompanyingchanges in pitch angle, or even the addition of ballast weights to thewing would provide a wider trench but in practice this would simplyprovide a deeper trench. Also an increase in thrust may mean that thewing has to be larger so as to counter-balance the upward reaction ofthe downward thrust.

A second proposal would be to increase the size of the wing dredger andprovide more thrust means. This would provide a more unwieldy and moreexpensive wing dredger. This is undesirable for the limited additionaluse that such a wing dredger would have for providing trenches ofgreater than normal width, and also because the wing dredger has to betransported, typically over land, between assignments. A known wingdredger already requires three lorries to move the various sections,and, for example, doubling in size would require six lorries to carryout the same transportation with attendant increased cost.

SUMMARY OF THE INVENTION

We will describe a wing dredger which may be used to provide a trench ofapproximately double the normal width.

In principle, the arrangement is such that the wing dredger is modifiedso as to provide the possibility that the dredger can be disposed at avariable selectable roll angle (ie it may be mounted at angles otherthan horizontal from side-to-side). In this way the wing dredger can bepassed over the line of the trench twice which will provide a doublewidth trench. The thrust means will be at an angle to the line of thetrench, so that the silt is blown first to one side of the trench andthen on the second pass to the opposite side of the trench.

The present invention provides, according to one aspect, a dredgingapparatus comprising a body mounting thrust means to direct, in use, awash of water downwards towards an area of sea bed or the like,connection means to connect said dredging apparatus to a support meansabove the sea bed, normally a support vessel floating on the water'ssurface, characterized in that said connection means included attitudeadjusting means to selectively adjust the attitude of the dredgingapparatus in a side to side (roll) orientation. Preferably said attitudeadjusting means is controllable to selectively adjust the attitude ofthe dredging apparatus independently in a front to rear (pitch) as wellas the side to side (roll) orientation.

Said attitude adjustable means may comprise at least three points onsaid body connected by flexible means to said support means, means beingprovided to independently vary the distance between the support meansand each said point.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described by way ofexample only and with reference to the accompanying drawings in which:

FIG. 1 is a diagrammatic view of a dredging apparatus in the form of aso called "wing dredger" in operation with its associated supportvessel,

FIG. 2 is a perspective view of the wing dredger of FIG. 1,

FIG. 3 is a cross section through the wing dredger of FIG. 2,

FIG. 4 is a diagrammatic front view of a wing dredger of FIG. 1. Innormal orientation and use,

FIGS. 5A and 5B show views similar to FIG. 4 of the wing dredger of theinvention in different orientations providing a trench of substantiallytwice the width of that provided in FIG. 4,

FIG. 6 shows one embodiment of attitude adjustable means in a wingdredger according to the invention,

FIGS. 7A-7C show side views and

FIGS. 8A-8C show front views of the wing dredger in different selectedattitudes,

FIGS. 9 and 10 show side and plan views respectively of an alternativearrangement of wing dredger, and

FIG. 11 shows a side view of the wing dredger of FIGS. 9 and 10 whenattached to the support vessel.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, a support vessel or mother vessel 10 is shownmoving forward or stationary heading into a tidal flow. The tidal flowmay be in a river, estuary, or at sea. To facilitate this operation, thearea to be covered can be buoyed along each side, or perhaps in channelsif a wide area is involved, to provide guide tracks for the vessel.Alternatively if the dredging operation is to be carried out over asmall area, the support vessel can be stationed over the area by a fourpoint mooring, or it can be dynamically positioned and the positioncontrolled by eg satellites. A wing dredger 11 is suspended at anappropriate distance from the sea bed via a pair of cables 12, 13, onecable extending from each side of a lifting means 14 on the mothervessel 10 and there is provided a further cable 16 from adjacent the bowof the vessel 10.

As illustrated in FIG. 2, the wing dredger has a hydrofoil cross sectionand is rectangular in plan. It is constructed as a casing comprisingvertical end walls 17, connected by laterally extending wall 18, whichprovide lower angled faces 19 to provide a downward component of forcewhen acted on by tidal flow providing as stability. The upper wall 21 isgenerally flat. Referring to FIG. 3, it will seen that the wing dredger11 is constructed of three units, a front unit 11A, and mid-unit 11B anda rear unit 11C connected together, so that the wing dredger can besplit into three sections for ease of transportation.

Each of the three units 11A, 11B and 11C are of steel skin constructionand units 11A and 11C are hollow closed boxes. The hollow closed boxesare divided into compartments by suitable bulkheads.

It will be seen from FIG. 3 that the wing dredger is symmetrical aboutits lateral axis so that it can be used in either direction with therespective end wall 18 leading. The dredger 11 is provided with twoclosed vertical bores 22 which are laterally spaced from each other,each housing a thrust means 23 in the form of a motor driven propeller24 mounted substantially in the plane of the wing 11 and the twopropellers are driven in opposition to reduce the effects ofcentrifugalicentripetal forces. Where the two contra-rotating verticaljet vortices meet, very high forces are created which increase seabedpenetration.

Upwardly extending. from the upper wall 21 are a pair of fins 26 and 27each extending from adjacent the front edge to adjacent the rear edge.Each closed vertical bore 22 extends up through a respective fin 26,27.The propellers are driven by respective electric motors.

In use, a downward vertical component of force is provided by theleading angled face 19 when acted on by the tide, and/or forward speedof the vessel, which component can be increased by adjustment of thecables to tilt the casing, and hence the upper wall 21 thereofappropriately to the horizontal.

In a practical construction capable of operating down to a depth ofapproximately 45 metres of water, the wing dredger has dimensions of theorder of 9000 mm×6000 mm. With a wing dredger of such size tilted 15degrees from the horizontal, a resultant hydro-dynamic downward verticalcomponent of force of up to about 9.5 tonnes is generated when the wingis being towed at 2 1/2 knots (4.6 km/hr) relative to the water and 12.5tonnes at 4 1/2 knots (8.3 km/hr). To counter the weight and forcecomponents the propellers are designed to produce a thrust of between0.5 and 5 tonne each.

Referring to FIG. 4, there is shown in a very diagrammatic form a frontview of a wing dredger as above described passing at a metre or twoabove the sea bed 29 and as a result of the downward thrust of thepropellers 24, there is produced a trench 31 in the sea bed 29. At leastsome of the material which has been dislodged from the sea bed toproduce the trench 31 is deposited on each side of the trench 31 to forma ridge 32.

In essence we provide a wing dredger in which instead of the side toside attitude of the wing dredger being horizontal as shown in FIG. 4,means may be provided whereby the lateral axis of the wing dredger maybe disposed at an angle to the horizontal. This angle is referred to asthe roll angle. Thus, in FIG. 5A in which the wing dredger is providedat a roll angle other than horizontal, the wing dredger may then providea trench 31A of the configuration shown in which the material from thetrench, some of which is deposited on the side of the trench, isdeposited so as to form a single ridge 32A on one side of the trench31A, and at a second passing shown in FIG. 5B, with the wing dredger 11at an opposite attitude or opposite roll angle, the trench 31A can bewidened so as to provide a trench 31B of approximately double the widthof the trench 31A and the material which is deposited on the secondpassing of the wing dredger over the trench, is deposited in a ridge 32Bon the opposite side to the ridge 32A.

We will now describe means whereby the roll angle of the wing dredger 11can be changed. We now refer to FIG. 6 which shows in schematic form themeans for mounting the wing dredger. The cables 12, 13, terminate attheir lower end in a pair of rings 41, 42 each ring being connected by arespective cable 43, 44, 46, 47 to fore and aft points 48-51 on thelateral side walls of the wing dredger 11. Ring 41 is connected by cable43 to the aft point of one side wall and by cable 44 to the fore pointof the same side wall and ring 42 is connected by cable 46 to the aftpoint of the other side wall and by cable 47 to the fore point of thesame side wall.

The fore and aft points 48-51 are actually provided by respectivewinches 52-55 the motors for which are mounted inside the wing dredger.Each winch 52-55 may be separately controlled.

It would be understood, therefore, that the side--side attitude (rollangle) of the wing dredger may be adjusted by suitably operating thewinches 52-55. For example, if it is desired to lift the end wall 17Awith respect to the opposite end wall 17B, then the winches 52, 53 maybe operated to wind in the cable 43, 44, thereby shortening theireffective length and lifting that end wall 17A. Alternatively, thewinches 54, 55 may be operated so as to unwind the cables 46, 47 tolengthen them or there may be a combination of shortening of cables 43,44 and lengthening of cables 46, 47 (as in FIG. 8B) as desired.

Similarly, to lift the end wall 17B with respect to end wall 1.7A (seeFIG. 8C), the winches will be operated in the opposite manner to shortencables 46, 47, and/or lengthen cables 43, 44.

It will also be observed that using the arrangement described in FIG. 6,it is possible change the attitude of the wing dredger 11 in the foreand aft direction, that is the pitch angle to an extent independently ofchanges of length of the cable 16. Thus, if is desired to lower thefront edge of the wing dredger, then the winches 51, 53 may be operatedto unwind the cables 44, 47 and thereby lengthen them and/or the winches52, 54 may be operated to wind in the cables 43, 46 to shorten them (seeFIG. 7B). Operation of the winches in the opposite manner will raise thefront of the wing dredger with respect to the rear, see FIG. 7C.

Other ways of changing the roll angle of the wing dredger may beprovided. For example, it may be sufficient to replace winches 52, 53 bya single central winch on which both cables 43, 44 are mounted, windingin both cables simultaneously to lift that end wall 17A, and a similararrangement being provided with respect to the opposite side of the wingdredger. On the other hand, such an arrangement would not allow one toadjust the pitch of the wing dredger.

We now refer to FIGS. 9, 10 and 11. It will be seen that the wingdredger 11 is mounted by means of the fore and aft points 48 to 51, bymeans of respective hydraulic rams 56 to 59 to rectangular frame 61located above the wing dredger. This rectangular frame 61 (known as aspreader) includes a pulley block 62, a cable 63 passing from the pulleyblock 62 to the lifting means 14 on the support vessel 10 (to be furtherdescribed with reference to FIG. 11).

It will understood, therefore, that to change the roll angle or pitchangle, in other words to change the attitude of the wing dredger 11laterally or fore and aft, it is necessary to change the relativelengths of the hydraulic rams 56-59 accordingly. Thus, contraction oframs 56, 57 and/or extension of rams 58, 59 will lift end wall 17A withrespect to their end wall 17B, contraction of rams 58, 59 and/orextension of rams 56, 57 will lift end wall 17B with respect to end wall17A. Contraction of rams 56, 58 and/or extension of rams 57, 59 willlift the rear of the wing dredger with respect to its front, andcontraction of rams 57, 59 and/or extension of rams 56, 58 will lift thefront of the wing dredger with respect to the rear.

Referring now to FIG. 11, there is shown a wing dredger, rectangularframe (spreader) 61 and pulley block 62 of FIGS. 9 and 10 but attachedto the lifting means 14 at the rear of the vessel 10. At the rear of thevessel 10 there are provided cantilever arms 66, 67 (which are primarilyprovided to keep the wing dredger away from the rear of the vessel), andthe pulley block 62 and rectangular frame 61 are connected to thecantilever arms 66, 67. In this configuration, the wing dredger ismounted rigidly to the rear of the vessel and may be disposed at anydesired transverse or fore to aft angle (any desired roll angle or pitchangle) for use in shallow water. In this configuration the wing dredgeris effectively firmly fixed to the mother vessel and the mother vesselmay be moved as desired so as to carry out whatever dredging operationis required.

The invention is not restricted to the details of the foregoingexamples. Various methods of selectively varying the roll angle of thewing dredger will occur to an expert in the field.

What is claimed is:
 1. A dredging apparatus comprising; a body having front and rear ends and opposite sides, a thruster mounted to said body to direct, in use, a wash of water downwards towards an area of seabed, connection structure to connect said dredging apparatus to a support vessel above the seabed, said body providing a face against which water flow caused by relative movement of the body and the water acts to provide a resultant downward component of force such that, in use, a weight of the body together with said resultant downward component of force provide a total downward force which exceeds an upward force provided by said thruster, said connection structure including adjustment means to selectively adjust the attitude of the dredging apparatus independently in a front to rear pitch orientation and a side to side roll orientation.
 2. A dredging apparatus as claimed in claim 1 wherein said adjustment means to selectively adjust the attitude comprises at least three points on said body connected by flexible means to said support vessel, and a length adjuster being provided to independently vary a distance between the support vessel and each said point.
 3. A dredging apparatus as claimed in claim 2 wherein said flexible means comprises an at least partially separate flexible means for each of said at least three points and said flexible means are connected to said support vessel by separate length adjusting means.
 4. A dredging apparatus as claimed in claim 1 in which the body is in the form of a wing comprising a casing having ballast tanks to adjust the weight thereof, said casing also having at least one closed bore passing between upper and lower faces, in which said thruster is located.
 5. A method of dredging in flowing water comprising the steps of: lowering a dredging apparatus comprising a body carrying a thruster so that the thruster is directed downwardly towards an area to be dredged, adjusting an orientation of the body in the water in a front to rear pitch orientation so that the body presents a surface relative to the flow which causes a resultant downward vertical component of force, operating said thruster to direct a wash of water towards the area, whereby turbulence created sets sand, silt and other material covering the area in suspension in the water so as to be carried away from the area by the flow of the water, and adjusting a roll orientation of the body in a side to side manner to cause the sand, silt and other material to be moved first to one side a subsequently to another side relative to the body.
 6. The method of claim 5 including conveying the body through the water in a first direction with the body roll orientation being adjusted to direct the sand, silt and other material away from the one side of the body and thereafter reversing the direction of the body and adjusting the body roll orientation to direct sand, silt and other material away from an opposite side of the body. 